Core D will perform the qualitative and quantitative histological and ultrastructural checks that are necessary to support all other Projects. The Core will not only supply various techniques of electron, phase contrast and confocal microscopy, but it will also operate at a high standard of quality and offer an expert critical evaluation of the results of experimental and molecular alterations. The uniform and consistent use of high quality images will allow the detection of even subtle alterations in protein-protein interactions and in the response of individual cell organelles that either are at the basis of altered functions or are the long term results of such alterations. It is clear that the overall ultrastructural response of the muscle fiber to mutations affecting excitation-contraction coupling are quite specific and offer considerable insight into causative effects. In the past period we have evidenced a strong fiber type- and gender-dependence of the pathology, that correspindes quite well with similar variations in function. Two general approaches are proposed. One is to define any alterations in the relationships between the major protein components of calcium release units (CRUs, triads in skeletal muscle) within the context of the mutation and the other is to follow the development of pathology (most specifically mitochondrial, myofibrillar and CRUs' alterations) through development and aging and in relation to the known functional effects of the mutation on CRUs' channels.This will be achieved by combining light microscope techniques (phase contrast of fibers whole mounts and confocal imaging of fluorescently immunolabeled fibers) with thin sectioning and freeze-fracture for electron microscopy supplemented by quantitative morphometry techniques. The core aims at defining the primary impact of each mutation on the macromelcular assembly of calcium release units within a short term and the secondary impact on SR, mitochondria and contractile material on the long term.